Stability in ecosystem functioning across a climatic threshold and contrasting forest regimes

Classical ecological theory predicts that changes in the availability of essential resources such as nitrogen should lead to changes in plant community composition due to differences in species-specific nutrient requirements. What remains unknown, however, is the extent to which climate change will alter the relationship between plant communities and the nitrogen cycle. During intervals of climate change, do changes in nitrogen cycling lead to vegetation change or do changes in community composition alter the nitrogen dynamics? We used long-term ecological data to determine the role of nitrogen availability in changes of forest species composition under a rapidly changing climate during the early Holocene (16k to 8k cal. yrs. BP). A statistical computational analysis of ecological data spanning 8,000 years showed that secondary succession from a coniferous to deciduous forest occurred independently of changes in the nitrogen cycle. As oak replaced pine under a warming climate, nitrogen cycling rates increased. Interestingly, the mechanism by which the species interacted with nitrogen remained stable across this threshold change in climate and in the dominant tree species. This suggests that changes in tree population density over successional time scales are not driven by nitrogen availability. Thus, current models of forest succession that incorporate the effects of available nitrogen may be over-estimating tree population responses to changes in this resource, which may result in biased predictions of future forest dynamics under climate warming.     

On the probability of reaching a threshold in a stochastic mammillary system

The multivariate distribution over time of a particular stochastic mammillary compartmental model is obtained for any point in time. The maximum expectation of the peripheral compartments is then derived and used to determine lower bounds on the probability that the maximum of the peripheral compartments reaches any arbitrary threshold level. A bound on the probability is illustrated by an example and some of its implications are explored.     

Puncture threshold prior to leakage from tissue expander reservoirs

An ex vivo study was devised to ascertain the maximum number of needle punctures possible prior to onset of leakage from remote-type tissue expansion reservoirs. At least 10 standard valves obtained from all major manufacturers were analyzed using various needle gauges. An 18-gauge beveled needle was determined to be the maximum size that could be safely employed for all vendors within a usual course for tissue expansion. No significant difference was found in comparison of Dacron-reinforced versus plain silicone membrane-type valves.     

The threshold of survival for system of two species in a polluted environment

The effects of toxicants on naturally stable two-species communities are studied. Persistence-extinction thresholds are given for populations in the toxicant stressed Lotka-Volterra model of two interacting species. The threshold results are expressed in terms of relationships involving the population intrinsic growth rates, dose-response parameters, and interaction rates.Research supported by the fund of Chinese Natural Science     

Measurement of the laser exposure levels for burn threshold in biological tissue

Experiments for the evaluation of the laser energy density required to induce burn threshold in biological tissue are presented. The results are compared with a theoretical model. The values obtained for soft tissue are higher than the pain threshold and the safety standards for the maximum permissible exposure. This is due to the different nature of injury associated with the surgical process.     

Tolerance and threshold in the extrinsic coagulation system

This paper focus on the quest for mechanisms that are able to create tolerance and an activation threshold in the extrinsic coagulation cascade. We propose that the interplay of coagulation inhibitor and blood flow creates threshold behavior. First we test this hypothesis in a minimal, four dimensional model. This model can be analysed by means of time scale analysis. We find indeed that only the interplay of blood flow and inhibition together are able to produce threshold behavior. The mechanism relays on a combination of raw substance supply and wash-out effect by the blood flow and a stabilization of the resting state by the inhibition. We use the insight into this minimal model to interpret the simulation results of a large model. Here, we find that the initiating steps (TF that produces together with fVII(a) factor Xa) does not exhibit threshold behavior, but the overall system does. Hence, the threshold behavior appears via the feedback loop (in that fIIa produces indirectly fXa that in turn produces fIIa again) inhibited by ATIII and blood flow.     

Stability of diacylglycerol acyltransferase in dehydrated bovine muscle tissue

Meaningful estimates of diacylglycerol acyltransferase (EC 2.3.1.20) activity in different tissue samples require effective, unbiased methods of sample storage. Samples of the pars costalis diaphragmatis muscle (skirt muscle of the diaphragm) were obtained from 18- to 20-month-old cattle and assayed for microsomal protein content and diacylglycerol acyltransferase activity after having been stored under various conditions as dissected tissue or microsomes prepared from dissected tissue. There was relative enrichment of diacylglycerol acyltransferase specific activity (p<0.05) when samples prepared from the pars costalis diaphragmatis muscle were dehydrated and stored for 2 weeks, as compared to the control condition (in which the microsome fraction was prepared from fresh pars costalis diaphragmatis muscle and assayed immediately). The results suggested that dehydration was an effective method of storage for bovine muscle samples destined for estimation of the microsomal diacylglycerol acyltransferase activity. The dehydration approach for preparing samples for analysis of diacylglycerol acyltransferase activity might also prove useful to investigators who are interested in obtaining reliable estimates of the activity of other enzymes in tissue samples.     

Stability of the saccadic oculomotor system

A variety of different types of instability has been found in the saccadic system of humans. Some of the instabilities correspond to clinical conditions, whereas others are inherent in the normal saccadic system. How can these instabilities arise within the mechanism of normal saccadic eye movements? A physiologically-based model of the saccadic system predicts that horizontal saccadic oscillations will occur with excessive mutual inhibition between the left and right burst cells and with underaction of the pause cells. The amplitudes and frequencies of the oscillations had ranges of 0–6° and 6–20 cycles per second, respectively. Application of stability analysis techniques to the model reveals that development of the oscillations can be explained by the Hopf bifurcation mechanism. Future development of this approach will involve classifying pathological instabilities of the saccadic system according to the bifurcation involved in their generation.     

Adipose tissue: a subtle and complex cell system

White adipose tissue (WAT) represents a large amount of all adult tissues. For a long time, it was considered as a poorly active, overgrown and undesirable tissue. It was mainly studied for its involvement in energy metabolism and disorders, as well as for its endocrine functions. WAT is composed of two main populations, matures adipocytes and stroma vascular fraction (SVF) that can be separated easily. The SVF contains two compartments, stromal and hematopoietic that have been recently characterized. The stromal population (or ADAS for Adipose Derived Stromal Cells) presents functional features of, as well as lineage relationship with, macrophages. These stromal cells, that are able to differentiate into adipocytes, also display endothelial potential, and could be considered as vascular progenitors. Differentiation of various adipose-derived cell subsets towards functional cardiomyocytes, osteoblasts, chondrocytes, muscle, hematopoietic and neural cells was also obtained in vitro or in vivo. Adipose tissue thus appears as a complex tissue composed of different cell subsets that could vary according to the nature and the location of fat pads, or to the physiological or pathological status. WAT appears as a very plastic and heterogeneous tissue that is very easy to sample. This represents a great advantage when considering adipose tissue as a potential and suitable source of stem cell for cell therapy. Further investigations in this way have to lead to the emergence of new insights fundamental to progress in our knowledge of adipose tissue biology.     

Virtual electrode theory explains pacing threshold increase caused by cardiac tissue damage

The virtual electrode polarization (VEP) effect is believed to play a key role in electrical stimulation of heart muscle. However, under certain conditions, including clinically, its existence and importance remain unknown. We investigated the influence of acute tissue damage produced by continuous pacing with strong current (40-mA, 4-ms biphasic pulses with 4-Hz frequency for 5 min) on stimulus-generated VEPs and pacing thresholds. A fluorescent optical mapping technique was used to obtain stimulus-induced transmembrane potential distribution around a pacing electrode applied to the ventricular surface of a Langendorff-perfused rabbit heart (n = 5). Maps and pacing thresholds were recorded before and after tissue damage. Spatial extents of electroporation and cell uncoupling were assessed by propidium iodide (n = 2) and connexin43 (n = 3) antibody staining, respectively. On the basis of these data, passive and active three-dimensional bidomain models were built to determine VEP patterns and thresholds for different-sized areas of the damaged region. Electrophysiological results showed that acute tissue damage led to disappearance of the VEP with an associated significant increase in pacing thresholds. Damage was expressed in electroporation and cell uncoupling within an approximately 1.0-mm-diameter area around the tip of the electrode. According to computer simulations, cell uncoupling, rather than electroporation, might be the direct cause of VEP elimination and threshold increase, which was nonlinearly dependent on the size of the damaged region. Fiber rotation with depth did not substantially affect the numerical results. The study explains failure to stimulate damaged tissue within the concepts of the VEP theory.     

Multiplication noise in the human visual system at threshold

Several kinds of light used in vision experiments produce photon statistics that are distinctly non-Poisson. Representative examples are light from a cathode-ray tube and an image-intensifier device. For the class of vision experiments in which the photon statistics play an important role, excess fluctuations produced by such light sources can alter the observed results and obscure the visual mechanisms being studied. They must therefore be accounted for in a proper way. We use the results of a Hecht-Shlaer-Pirenne type experiment, carried out with modulated Poisson light, to illustrate the point. Sensitivity and modulation depth, as well as sensitivity and reliability, are shown to be traded against each other. Finally, we demonstrate that number-state light, which is comprised of photons of an ideal kind, provides the ultimate tool for extracting information about the intrinsic noise distribution in the visual system at threshold. The state of the art in producing such light is discussed.This work was carried out at Columbia University and was supported by the National Science Foundation and the National Institutes of Health     

Stability of a quasi-flute mode under the Bernstein-Kadomtsev condition in a toroidal confinement system

It is shown that the growth rate of the MHD instability in toroidal configurations is slower in a situation in which the Bernstein-Kadomtsev condition is satisfied while the Mercier stability criterion is not. Under the Bernstein-Kadomtsev condition, Alfvénic Mercier modes are not excited, but quasi-flute acoustic Mercier modes develop instead. In confinement systems with closed magnetic field lines, the Bernstein-Kadomtsev condition ensures MHD stability; however, a small rotational transform produced by magnetic perturbations can give rise to a quasi-flute acoustic instability whose growth rate is proportional to the perturbation amplitude, in which case the fastest growing oscillations are those with the shortest wavelengths.     

Wetting, percolation and morphogenesis in a model tissue system

Artificial tissues constructed of cells or polystyrene beads suspended in a solution of type I collagen will, under appropriate conditions, protrude into regions of similar matrices lacking particles, but containing the extracellular glycoprotein fibronectin. This phenomenon has been termed "matrix-driven translocation". Conditions required for the effect include the presence of heparin-like molecules on the cell or bead surfaces, appropriate concentrations of particles and collagen, and physiological ionic strength and pH. Here we consider the idea that the driving force for the concerted movement of matrix and suspended particles is the thermodynamically spontaneous spreading or wetting behavior of two immiscible fluids bounded by common substrata. Wetting theory is shown to be capable of accounting for the behavior of this model system, but this analysis requires that the two matrix regions constitute separate phases at thermodynamic coexistence. We show that one plausible mechanism for the generation of separate phases is the formation of a percolation network of collagen fibers on a lattice of cells or beads. It is argued that the concepts of wetting and percolation apply to properties in common between the model system and living tissues, and may therefore be used to provide a physical account of aspects of tissue morphogenesis.     

Molecular characterization of a local sulfonylurea system in human adipose tissue

ATP-sensitive potassium (KATP) channels are present in many cell types and link cellular metabolism to the membrane potential. These channels are heterooctamers composed of two subunits. The sulfonylurea receptor (SUR) subunits are targets for drugs that are inhibitors or openers of the KATP channels, while the inwardly rectifying K+ (Kir) subunits form the ion channel. Two different SUR genes (SUR1 and SUR2) and two different Kir6.x genes (Kir6.1 and Kir6.2) have been identified. In addition, isoforms of SUR2, SUR2A and SUR2B, have been described. We have previously performed expression profiling on pooled human adipose tissue and found high expression of SUR2. Others have reported expression of SUR1 in human adipocytes. The aim of this study was to characterize the expression of the sulfonylurea receptor complex components in human adipose tissue. RT-PCR analysis, verified by restriction enzyme digestions and DNA sequencing, showed that SUR2B, Kir6.1 and alpha-endosulfine, but not SUR1, SUR2A or Kir6.2, are expressed in human adipose tissue. Real-time RT-PCR showed that SUR2B was expressed at higher levels in subcutaneous compared with omental adipose tissue in paired biopsies obtained from seven obese men (p < 0.05). Analysis of tissue distribution showed that SUR2B expression in adipose tissue was lower than that in muscle, similar to that in heart and liver, while the expression in pancreas was lower. The effect of caloric restriction was tested in obese men (n = 10) treated with very low calorie diet for 16 weeks, followed by a gradual reintroduction of ordinary food for 2 weeks. Biopsies were taken at week 0, 8 and 18. There was no consistent effect of weight reduction on SUR2B or Kir6.1 expression. We conclude that the necessary components for a local sulfonylurea system are expressed in human adipose tissue and that the sulfonylurea receptor complex in this tissue is composed of SUR2B and Kir6.1. The expression of SUR2B was higher in subcutaneous compared with omental adipose tissue and was not affected by weight loss.     

Transplantation of engineered bone tissue using a rotary three-dimensional culture system

Bone is a complex, highly structured, mechanically active, three-dimensional (3-D) tissue composed of cellular and matrix elements. We previously published a report on in situ collagen gelation using a rotary 3-D culture system (CG–RC system) for the construction of large tissue specimens. The objective of the current study was to evaluate the feasibility of bone tissue engineering using our CG–RC system. Osteoblasts from the calvaria of newborn Wistar rats were cultured in the CG–RC system for up to 3 wk. The engineered 3-D tissues were implanted into the backs of nude mice and calvarial round bone defects in Wistar rats. Cell metabolic activity, mineralization, and bone-related proteins were measured in vitro in the engineered 3-D tissues. Also, the in vivo histological features of the transplanted, engineered 3-D tissues were evaluated in the animal models. We found that metabolic activity increased in the engineered 3-D tissues during cultivation, and that sufficient mineralization occurred during the 3 wk in the CG–RC system in vitro. One mo posttransplantation, the transplants to nude mice remained mineralized and were well invaded by host vasculature. Of particular interest, 2 mo posttransplantation, the transplants into the calvarial bone defects of rats were replaced by new mature bone. Thus, this study shows that large 3-D osseous tissue could be produced in vitro and that the engineered 3-D tissue had in vivo osteoinductive potential when transplanted into ectopic locations and into bone defects. Therefore, this system should be a useful model for bone tissue engineering.     

Parameter estimation of the threshold time function in the neural system

An algorithm for parameter estimation is presented for the neural system model. Because of its firing mechanism analogous to that of the model based on the first time crossing problem, this problem is solved numerically for our model according to the results of Kostyukov et al. (1981). We propose the algorithm that estimates the parameters of the model considering the equivalence between the probability density function of the 1st crossing time and that of the interspike interval, which is derived from the interspike interval histogram by making use of the spline function technique. The ability of the algorithm is ensured by the application to the simulated interspike interval data. The parameter estimation is carried out also for the practical neural data recorded in the cat's optic tract fibers in both the spontaneous and the stimulated cases. These applications will show the effectiveness of the algorithm in practical cases.